1. Field of the Invention
The invention relates to signal processing, and more particularly to an array microphone.
2. Description of the Related Art
The Delta-Sigma (ΔΣ) modulation is a kind of analog-to-digital signal conversion derived from delta modulation. An analog to digital converter (ADC) circuit which implements this technique can be easily realized using low-cost CMOS processes. The benefit of a delta-sigma converter is that it moves most of the conversion process into the digital domain. This makes it easier to combine high-performance analog with digital processing.
FIG. 1 shows an example of analog-to-digital converter 120 for converting an analog signal to a digital one-bit datastream. The analog-to-digital converter 120 includes a multi-bit delta-sigma modulator 104 and a one-bit delta-sigma modulator 106. A sound wave in the air is received by a microphone module 102 and converted to an analog signal. For further processing, the analog-to-digital converter 120 then converts the analog signal to a digital one-bit datastream. The multi-bit delta-sigma modulator 104 first converts the analog signal to a multi-bit datastream, which is not as sensitive to clock jitters as the one-bit datastream. The one-bit delta-sigma modulator 106 then converts the multi-bit datastream to the one-bit datastream.
Both the multi-bit delta-sigma modulator 104 and the one-bit delta-sigma modulator 106 are triggered by the same clock signal. Because the one-bit delta-sigma modulator 106 is a pure digital modulator, the jitters of the the clock signal make no impact on the one-bit datastream generated by the one-bit delta-sigma modulator 106. Thus, although triggered by a clock signal, the analog-to-digital converter 120 can better avoid clock jitter interference than a single one-bit delta-sigma modulator which directly converts the analog signal to a one-bit datastream.
With the increased maturity in speech and speaker processing technologies, and the prevalence of telecommunications, there is a need for effective speech acquisition devices. Microphone arrays have a distinct advantage as they enable hands-free acquisition of speech with little constraint on the user, and they can also provide information on the location of speakers. A microphone array consists of multiple microphones at different locations. Using sound propagation principles, the individual microphone signals can be filtered and combined to enhance sound originating from a particular direction or location. The location of the principal sound sources can also be determined dynamically by investigating the correlation between different microphone channels.
FIG. 2 is a block diagram of a portion of a signal processing device 200, which includes a microphone array 210 for acquiring sound waves. The microphone array 210 includes two microphone modules 202 and 212, which are oriented towards different directions of the signal processing device 200. The microphone modules 202 converts a left sound wave SL to an analog signal AL, and the microphone modules 212 converts a right sound wave SR to an analog signal AR. The analog-to-digital converters 208 and 218 then respectively convert the analog signals AL and AR to digital one-bit datastreams DL and DR. Both of the analog-to-digital converters 208 and 218 have the same composition as the analog-to-digital converter 120 of FIG. 1, and respectively includes a multi-bit delta-sigma modulator and a one-bit delta-sigma modulator connected in cascade. The one-bit delta-sigma modulators 204 and 214 converts analog signals AL and AR to multi-bit datastreams ML and MR. The one-bit delta-sigma modulators 206 and 208 then convert the multi-bit datastreams ML and MR to one-bit datastreams DL and DR.
The signal processing device 200, however, lacks the ability to mix signals. Because the analog-to-digital converters 208 and 208 respectively includes a multi-bit delta-sigma modulator and a one-bit delta-sigma modulator, the multi-bit datastreams output by the multi-bit delta-sigma modulators can be further processed by a mixer to generate the input datastreams of the one-bit delta-sigma modulators. Thus, the number of the one-bit delta-sigma modulators can be reduced, and an analog-to-digital converter capable of mixing signals is introduced.